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4.0 EXPOSURE ASSESSMENT The exposure assessment describes the magnitude and duration of exposure of receptor populations to chenucals at the site The process of estimating exposure consists of identifying (1) potentially-exposed individuals, (2) the exposure pathways occurring (or reasonably expected to occur) at the site, (3) estimates of chemical exposure point concentrations, (4) the parameters which quantitatively describe dose, and (5) the uncertainty in the parameters T ils section is divided into five subsections Subsection 4 1 describes the exposure assessment process Subsection 4 2 discusses the site conditions assumed for the assessment Subsection 4 3 describes exposure pathways and potentially-exposed populations Subsection 4 4 describes the subsurface migration of petroleum hydrocarbons in soil at the site using environmental fate and transport modeling Section 4 5 presents a summary of the exposure assessment 4.1 Exposure Assessment Process The general procedure for conducting an exposure assessment is illustrated in FIGURE 4-1 taken from the USEPA's risk assessment guidance document(USEPA, 1989) The exposure assessment proceeds with the following steps Step 1 Characterization of Exposure Setting - The exposure setting is characterized with respect to the general physical characteristics of the site and the characteristics of the populations on and/or near the site Step 2 Identification of Exposure Pathways - The exposure pathways by which the previously identified populations may be exposed are selected in this step Step 3 Quantification of Exposure - The magnitude, frequency, and duration of exposure for each pathway identified in Step 2 is quantified This step usually consists of two stages (1) estimation of exposure point concentrations and (2) calculation of doses Step 1 has been presented in Section 2.0 Steps 2 and 3 are presented in this section G�ft=IMAWAFE FNL\kuW 26 93 4-1 .r. 19"